1. Field of the Invention:
This invention relates to the generation of x-rays particularly useful in x-ray lithographic systems to obtain a uniform x-ray flux emanating from a small spot. Such systems may be used in the fabrication of wafers for large scale integrated circuit production. The invention may also be utilized in x-ray sources for medical radiographic x-ray, diffraction study and tomographic applications. More particularly, it relates to means to more effectively cool the target anode and to prevent burn-out of the conical tip of prior art anodes.
2. Description of the Prior Art
X-rays are utilized in various fields including medical imaging and x-ray lithographic systems for fabricating large scale integration (LSI) semiconductor devices. In certain types of systems the X-rays are formed by an annular-shaped electron gun, various shield and extraction grids and beam focusing devices to effect transmission of a hollow high-energy (about 10-30 keV, typically 25 keV) electron beam on an inverted conical-shaped target. The vast majority, i.e. 99-99.9% of the energy in the beam of electrons bombarding the target, is converted to heat energy but most of the remaining energy produce x-rays which exit through an x-ray transmissive window (typically beryllium for lithography applications) which is part of the enclosure of the overall vacuum system. Generators of this type are disclosed in U.S. Pat. No. 3,665,236 (Gaines et al.), U.S. Pat. No. 3,892,989 (Gralenski et al.), and in Nuclear Instruments and Methods 126 (1975) pages 99-101. Improvements particularly to the cooling of the conical shaped anode are seen in U.S. Pat. Nos. 4,238,682 and 4,258,262 and in the Journal Vacuum Science Technology 16(6) Nov./Dec. 1979, pages 1942-1945 wherein a gun similar to the Gaines et al. electron gun is utilized. Cooling of the target anode is effected by providing a water diverter to provide high water velocity on the back side of the inverted conical anode target so as to establish high velocity turbulent flow resulting in nucleate boiling. This avoids laminar flow or vapor layers forming on that cone back surface. The authors of the Journal Vacuum Science Technology article who include the inventors of the cited improvement patents caution to avoid heating the apex of the cone with the electron beam because to do so will burn out the tip of the cone, since the apex of the cone is not efficiently cooled.
In most applications of x-rays, the greater the intensity of the x-rays, the shorter the exposure. To generate more x-rays, more electrons are pulled from the cathode to impinge on the target. However, the more electrons impinging on the target, the more heat that must be removed from the target. In certain applications the target is rotated to prevent heat from building up. But this results in a complicated structure. In medical x-rays, high power is required over a short period to yield a low average power. In lithography, a high average power is used because the x-ray source is on continuously or almost continuously. Thus, the removal of heat from the conical target is critical.
Electrons hitting the anode target surface penetrate a few microns into the surface. Heat is transferred to the cool side of the target anode where it is removed by high speed coolant, normally water, passing the cool-side surface. Such high speed collant flow is not possible on the apex of the prior art conical target no effort is made at all costs to avoid beam infringement adjacent to or on the apex.